Coiled tubing drilling requires the use of a down hole positive displacement motor (PDM) to rotate the drill bit. During drilling operations, the unloaded PDM rotates at a constant RPM and achieves a free spin motor pressure with respect to the fluid flow rate. As the drill bit encounters the bottom of the hole and forces are transferred to the bit, referred to as (WOB), the motor will register an increase in torque. This increase in torque is a result of increased resistance to rotating at the constant RPM assuming a constant flow rate of drilling fluid. In return, the PDM requires additional pressure to turn the motor at the constant RPM while under increased resistance. If the resistance increases to a condition which prohibits the PDM from rotating, a motor stall is encountered. During a motor stall, the motor stops turning, the down hole fluid path is severely restricted, and the surface pump pressure dramatically increases. This event can eventually cause a motor failure, which requires the drilling process to be stopped.
Time constraints and the resultant daily operational costs are always a consideration for any down hole drilling operation. High drilling rates of penetration (ROP) coupled with minimal delays results in a highly desirable, cost-effective operation. Overly optimistic drilling rates of penetration in difficult formations place undue strain on sensitive down hole motors and can lead to premature failure of the rubber stator. These failures are expensive and time consuming as they require additional trips to surface as well as motor replacement.
A tool specifically designed to prevent over pressure scenarios for down hole drilling motors having a rotor and a stator arrangement would allow operators to achieve higher rates of penetration while still maintaining drilling motor differential pressures within manufacturer specification. Such an improvement would have tangible results in the pursuit of a highly efficient and cost-effective operation. In order for a down hole tool to protect a drilling motor from over pressure scenarios it must also be able to maintain differential pressures within manufacturer specification, regardless of operator input parameters controlling the rate of penetration. Consequently, a need exists for a down hole tool which can protect the drilling motor from over pressure scenarios while simultaneously allowing the motor to operate within manufacturer specifications. The present invention provides a down hole tool to achieve this objective.